Hybrid (biology)

In biology a hybrid also known as cross breed is the mixing, through sexual reproduction, of two animals or plants of different breeds, varieties, species or genera.[1] Using genetics terminology, it may be defined as follows.[2]

In general usage, hybrid is synonymous with heterozygous: any offspring resulting from the breeding of two genetically distinct individuals

Hybrids between different subspecies within a species (such as between the Bengal tiger and Siberian tiger) are known as intra-specific hybrids. Hybrids between different species within the same genus (such as between lions and tigers) are sometimes known as interspecific hybrids or crosses. Hybrids between different genera (such as between sheep and goats) are known as intergeneric hybrids. Extremely rare interfamilial hybrids have been known to occur (such as the guineafowl hybrids).[4] No interordinal (between different orders) animal hybrids are known.

The third type of hybrid consists of crosses between populations, breeds or cultivars within a single species. This meaning is often used in plant and animal breeding, where hybrids are commonly produced and selected, because they have desirable characteristics not found or inconsistently present in the parent individuals or populations.

The term hybrid is derived from Latin hybrida, meaning the "offspring of a tame sow and a wild boar", "child of a freeman and slave", etc.[5] The term came into popular use in English in the 19th century, though examples of its use have been found from the early 17th century.[6]

Depending on the parents, there are a number of different types of hybrids;[7]

Single cross hybrids — result from the cross between two true breeding organisms and produces an F1 generation called an F1 hybrid (F1 is short for Filial 1, meaning "first offspring"). The cross between two different homozygous lines produces an F1 hybrid that is heterozygous; having two alleles, one contributed by each parent and typically one is dominant and the other recessive. Typically, the F1 generation is also phenotypically homogeneous, producing offspring that are all similar to each other.

Double cross hybrids — result from the cross between two different F1 hybrids.[8]

Three-way cross hybrids — result from the cross between one parent that is an F1 hybrid and the other is from an inbred line.[9]

Triple cross hybrids — result from the crossing of two different three-way cross hybrids.

Population hybrids — result from the crossing of plants or animals in a population with another population. These include crosses between organisms such as interspecific hybrids or crosses between different breeds.

Interspecific hybrids are bred by mating two species, normally from within the same genus. The offspring display traits and characteristics of both parents. The offspring of an interspecific cross are very often sterile; thus, hybrid sterility prevents the movement of genes from one species to the other, keeping both species distinct.[12] Sterility is often attributed to the different number of chromosomes the two species have, for example donkeys have 62 chromosomes, while horses have 64 chromosomes, and mules and hinnies have 63 chromosomes. Mules, hinnies, and other normally sterile interspecific hybrids cannot produce viable gametes, because differences in chromosome structure prevent appropriate pairing and segregation during meiosis, meiosis is disrupted, and viable sperm and eggs are not formed. However, fertility in female mules has been reported with a donkey as the father.[13]

Most often other processes occurring in plants and animals keep gametic isolation and species distinction. Species often have different mating or courtship patterns or behaviors, the breeding seasons may be distinct and even if mating does occur antigenic reactions to the sperm of other species prevent fertilization or embryo development. Hybridisation is much more common among organisms that spawn indiscriminately, like soft corals and among plants.

While it is possible to predict the genetic composition of a backcross on average, it is not possible to accurately predict the composition of a particular backcrossed individual, due to random segregation of chromosomes. In a species with two pairs of chromosomes, a twice backcrossed individual would be predicted to contain 12.5% of one species' genome (say, species A). However, it may, in fact, still be a 50% hybrid if the chromosomes from species A were lucky in two successive segregations, and meiotic crossovers happened near the telomeres. The chance of this is fairly high: (where the "two times two" comes about from two rounds of meiosis with two chromosomes); however, this probability declines markedly with chromosome number and so the actual composition of a hybrid will be increasingly closer to the predicted composition.

Hybrids are often named by the portmanteau method, combining the names of the two parent species. For example, a zeedonk is a cross between a zebra and a donkey. Since the traits of hybrid offspring often vary depending on which species was mother and which was father, it is traditional to use the father's species as the first half of the portmanteau. For example, a liger is a cross between a male lion and a female tiger, while a tigon is a cross between a male tiger and a female lion.

While not very common, a few animal species have been recognized as being the result of hybridization. The Lonicera fly is an example of a novel animalspecies that resulted from natural hybridization. The American red wolf appears to be a hybrid species between gray wolf and coyote,[14] although its taxonomic status has been a subject of controversy.[15][16][17] The European edible frog appears to be a species, but is actually a semi-permanent hybrid between pool frogs and marsh frogs. The edible frog population is dependent on the presence of at least one of the parents species to be maintained.[18]

Hybrid species of plants are much more common than animals. Many of the crop species are hybrids, and hybridization appear to be an important factor in speciation in some plant groups.

Beefalo, a cross of an American bison and a domestic cow. This is a fertile breed; this along with genetic evidence has caused them to be recently reclassified into the same genus, Bos.[citation needed]

Bengal cat, a cross between the Asian leopard cat and the domestic cat, one of many hybrids between the domestic cat and wild cat species. The domestic cat, African wild cat and European wildcat may be considered variant populations of the same species (Felis silvestris), making such crosses non-hybrids.

In Australia, New Zealand and other areas where the Pacific Black Duck occurs, it is hybridised by the much more aggressive introduced Mallard. This is a concern to wildlife authorities throughout the affected area, as it is seen as Genetic pollution of the Black Duck gene pool.

Saltwater crocodiles (Crocodylus porosus) have mated with Siamese crocodiles (Crocodylus siamensis) in captivity producing offspring which in many cases have grown over 20 feet (6.1 metres) in length. It is likely that wild hybridization occurred historically in parts of southeast Asia.

Many species of boas and pythons are known to produce hybrids, such as carball (a cross between a ball python and a carpet python) or a bloodball (a cross between a blood python and a ball python) however, most of these only occur in captivity. Contrary to popular belief, boa–python hybrids are not possible due to their differing reproductive functions. Boas only produce hybrids with other species of boas, and pythons only produce hybrids with other species of pythons.

Killer bees were created during an attempt to breed a strain of bees that would produce more honey and be better adapted to tropical conditions. This was done by crossing a European honey bee and an African bee.

Many hybrids are created by humans, but natural hybrids occur as well. Plant species hybridize more readily than animal species, and the resulting hybrids are more often fertile hybrids and may reproduce, though there still exist sterile hybrids and selective hybrid elimination where the offspring are less able to survive and are thus eliminated before they can reproduce. A number of plant species are the result of hybridization and polyploidy with many plant species easily cross pollinating and producing viable seeds, the distinction between each species is often maintained by geographical isolation or differences in the flowering period. Since plants hybridize frequently without much work, they are often created by humans in order to produce improved plants. These improvements can include the production of more or improved seeds, fruits or other plant parts for consumption, or to make a plant more winter or heat hardy or improve its growth and/or appearance for use in horticulture. Much work is now being done with hybrids to produce more disease resistant plants for both agricultural and horticultural crops. In many groups of plants hybridization has been used to produce larger and more showy flowers and new flower colors.

Many plant genera and species have their origins in polyploidy. Autopolyploidy results from the sudden multiplication in the number of chromosomes in typical normal populations caused by unsuccessful separation of the chromosomes during meiosis. Tetraploids (plants with four sets of chromosomes rather than two) are common in a number of different groups of plants and over time these plants can differentiate into distinct species from the normal diploid line. In Oenothera lamarchiana the diploid species has 14 chromosomes, this species has spontaneously given rise to plants with 28 chromosomes that have been given the name Oenothera gigas. When hybrids are formed between the tetraploids and the diploid population, the resulting offspring tend to be sterile triploids, thus effectively stopping the intermixing of genes between the two groups of plants (unless the diploids, in rare cases, produce unreduced gametes).

Another form of polyploidy called allopolyploidy occurs when two different species mate and produce polyploid hybrids. Usually the typical chromosome number is doubled, and the four sets of chromosomes can pair up during meiosis, thus the polyploids can produce offspring. Usually, these offspring can mate and reproduce with each other but cannot back-cross with the parent species. Allopolyploids may be able to adapt to new habitats that neither of their parent species inhabited.

Sterility in a non-polyploid hybrid is often a result of chromosome number; if parents are of differing chromosome pair number, the offspring will have an odd number of chromosomes, leaving them unable to produce chromosomally balanced gametes.[27] While this is undesirable in a crop such as wheat, where growing a crop which produces no seeds would be pointless, it is an attractive attribute in some fruits. Triploidbananas and watermelons are intentionally bred because they produce no seeds (and are parthenocarpic).

Hybrids are sometimes stronger than either parent variety, a phenomenon most common with plant hybrids, which when present is known as hybrid vigor (heterosis) or heterozygote advantage.[28] A transgressive phenotype is a phenotype displaying more extreme characteristics than either of the parent lines.[29]Plant breeders make use of a number of techniques to produce hybrids, including line breeding and the formation of complex hybrids. An economically important example is hybrid maize (corn), which provides a considerable seed yield advantage over open pollinated varieties. Hybrid seed dominates the commercial maize seed market in the United States, Canada and many other major maize producing countries.[30]

The multiplication symbol × (not italicised) indicates a hybrid in the Latin binomial nomenclature. Placed before the binomial it indicates a hybrid between species from different genera (intergeneric hybrid):-

Hybridization between two closely related species is actually a common occurrence in nature but is also being greatly influenced by anthropogenic changes as well.[31] Hybridization is a naturally occurring genetic process where individuals from two genetically distinct populations mate.[32] As stated above, it can occur both intraspecifically, between different distinct populations within the same species, and interspecifically, between two different species. Hybrids can be either sterile/not viable or viable/fertile. This affects the kind of effect that this hybrid will have on its and other populations that it interacts with.[33] Many hybrid zones are known where the ranges of two species meet, and hybrids are continually produced in great numbers. These hybrid zones are useful as biological model systems for studying the mechanisms of speciation (Hybrid speciation). Recently DNA analysis of a bear shot by a hunter in the North West Territories confirmed the existence of naturally-occurring and fertile grizzly–polar bear hybrids.[34]

Humans have introduced species world wide to environments for a long time, both intentionally such as establishing a population to be used as a biological control, and unintentionally such as accidental escapes of individuals out of agriculture. This causes drastic global effects on various populations, including through hybridization.[33][36]

When habitats become broken apart, one of two things can occur, genetically speaking. The first is that populations that were once connected can be cut off from one another, preventing their genes from interacting. Occasionally, this will result in a population of one species breeding with a population of another species as a means of surviving such as the case with the red wolves. Their population numbers being so small, they needed another means of survival. Habitat fragmentation also led to the influx of generalist species into areas where they would not have been, leading to competition and in some cases interbreeding/incorporation of a population into another. In this way, habitat fragmentation is essentially an indirect method of introducing species to an area.

There is a kind of continuum with three semi-distinct categories dealing with anthropogenic hybridization: hybridization without Introgression, hybridization with widespread introgression, and essentially a Hybrid swarm.[31] Depending on where a population falls along this continuum, the management plans for that population will change. Hybridization is currently an area of great discussion within Wildlife management and habitat management. Global climate change is creating other changes such as difference in population distributions which are indirect causes for an increase in anthropogenic hybridization.

Hybridization can be a less discussed way toward extinction than within detection of where a population lies along the hybrid continuum. The dispute of hybridization is how to manage the resulting hybrids. When a population experiences hybridization with substantial introgression, there still exists parent types of each set of individuals. When a complete hybrid swarm is created, all the individuals are hybrids.[citation needed]

Conservationists disagree on when is the proper time to give up on a population that is becoming a hybrid swarm or to try and save the still existing pure individuals. Once it becomes a complete mixture, we should look to conserve those hybrids to avoid their loss.[31] Most leave it as a case-by-case basis, depending on detecting of hybrids within the group. It is nearly impossible to regulate hybridization via policy because hybridization can occur beneficially when it occurs "naturally" and there is the matter of protecting those previously mentioned hybrid swarms because if they are the only remaining evidence of prior species, they need to be conserved as well.[31]

When two distinct types of organisms breed with each other, the resulting hybrids typically have intermediate traits (e.g., one parent has red flowers, the other has white, and the hybrid, pink flowers).[37] Commonly, hybrids also combine traits seen only separately in one parent or the other (e.g., a bird hybrid might combine the yellow head of one parent with the orange belly of the other).[37]

In a hybrid, any trait that falls outside the range of parental variation is termed heterotic. Heterotic hybrids do have new traits, that is, they are not intermediate. Positive heterosis produces more robust hybrids, they might be stronger or bigger; while the term negative heterosis refers to weaker or smaller hybrids.[38] Heterosis is common in both animal and plant hybrids. For example, hybrids between a lion and a tigress ("ligers") are much larger than either of the two progenitors, while a tigon (lioness × tiger) is smaller. Also the hybrids between the Common Pheasant (Phasianus colchicus) and domestic fowl (Gallus gallus) are larger than either of their parents, as are those produced between the Common Pheasant and hen Golden Pheasant (Chrysolophus pictus).[39] Spurs are absent in hybrids of the former type, although present in both parents.[40]

Regionally developed ecotypes can be threatened with extinction when new alleles or genes are introduced that alter that ecotype. This is sometimes called genetic mixing.[41] Hybridization and introgression of new genetic material can lead to the replacement of local genotypes if the hybrids are more fit and have breeding advantages over the indigenous ecotype or species. These hybridization events can result from the introduction of non native genotypes by humans or through habitat modification, bringing previously isolated species into contact. Genetic mixing can be especially detrimental for rare species in isolated habitats, ultimately affecting the population to such a degree that none of the originally genetically distinct population remains.[42][43]

In agriculture and animal husbandry, the Green Revolution's use of conventional hybridization increased yields by breeding "high-yielding varieties". The replacement of locally indigenous breeds, compounded with unintentional cross-pollination and crossbreeding (genetic mixing), has reduced the gene pools of various wild and indigenous breeds resulting in the loss of genetic diversity.[44] Since the indigenous breeds are often well-adapted to local extremes in climate and have immunity to local pathogens this can be a significant genetic erosion of the gene pool for future breeding. Therefore, commercial plant geneticists strive to breed "widely adapted" cultivars to counteract this tendency.[45]

A number of conditions exist that limit the success of hybridization, the most obvious is great genetic diversity between most species. But in animals and plants that are more closely related hybridization barriers can include morphological differences, differing times of fertility, mating behaviors and cues, physiological rejection of sperm cells or the developing embryo.[citation needed]

In plants, barriers to hybridization include blooming period differences, different pollinator vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and structural differences of the chromosomes.[46]

Ancient folktales often contain mythological creatures, sometimes these are described as hybrids (e.g., Hippogriff as the offspring of a griffin and a horse, and the Minotaur which is the offspring of Pasiphaë and a white bull). More often they are kind of chimera, i.e., a composite of the physical attributes of two or more kinds of animals, mythical beasts, and often humans, with no suggestion that they are the result of interbreeding, e.g., Harpies, mermaids, and centaurs.

In the Bible, the Old Testament contains several passages which talk about a first generation of hybrid giants who were known as the Nephilim.[47] The Book of Genesis (6:4) states that "the sons of God went to the daughters of humans and had children by them". As a result, the offspring was born as hybrid giants who became mighty heroes of old and legendary famous figures of ancient times.[48] In addition, the Book of Numbers (13:33) says that the descendants of Anak came from the Nephilim, whose bodies looked exactly like men, but with an enormous height. According to the apocryphalBook of Enoch the Nephilim were wicked sons of fallen angels who had lusted with attractive women.[49]